Shoe press belt

ABSTRACT

Nip press belt of a wet press or a calender with elongated nip, in particular for a paper, cardboard or tissue machine, with a flexible elastomer layer that is impermeable to liquids and with a textile reinforcing layer, wherein the elastomer layer comprises soft rubber with a hardness in the range between 5 and 100 P+J.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 371 from PCTApplication No. PCT/EP02/03129, filed on Mar. 20, 2002, which claims thebenefit of European Application Serial No. 01 109 618.7 filed on Apr.18, 2001, the disclosures and contents of which are incorporated byreference herein in their entireties.

DESCRIPTION

The invention relates to a nip press belt according to theprecharacterizing clause of claim 1.

A nip press belt of this kind, in German also called Preβmantel(translatable as “press jacket”), is used in paper, cardboard or tissuemachines to remove most of the water from the product concerned (wetpress) or to finish the surface (calender). Such presses or calendershave an elongated gap (“nip”) for passage of the product and aretherefore also called “nip presses”.

Within the nip the press belt makes sliding contact, by way of its back(lower) surface, with the actual pressing element; therefore this backsurface must have excellent sliding properties. On its front (upper)surface the press belt makes contact either with a pulp felt (by way ofwhich it indirectly contacts the product concerned) or—in the case of acalender—directly with the product, against which it is pressed by arotating roller.

In addition to the above-mentioned sliding properties of the backsurface, another important factor is the impermeability of the belt toliquid, because water should not pass from the product or pulp felt tothe pressing element, nor may lubricant from the latter enter theproduct or the felt. Furthermore, certain elasticity and flexibilitycharacteristics are required.

Nip press belts of various designs are known in the state of the art.

For example, the patent EP 0 420 372 B1 describes a nip press belt ofthis generic kind with a basic web in the form of an endless loopcovered on its inner and outer surfaces with a smooth polymer-resincoating, which makes the web impermeable to liquids and endows it with auniform thickness. The polymer resin that forms the elastomer is herespecified as polyurethane.

The patent DE 40 28 085 C1 discloses another band for use in papermachines, in particular wet presses with elongated nip (nip presses).The band has on its back surface a smooth, flexible band layer that isimpermeable to liquids, and on the front surface there is a carriertread with a fiber layer attached thereto.

The patent DE 42 02 731 A1 also discloses a belt, here termed “jacket”,of this generic kind for a nip press, which comprises an elastomericjacket material and two layers of reinforcing threads. As specificationof the jacket material, reference is made here on one hand to materialcapable of swelling, and on the other hand to polyurethane.

In WO 95/16820 a paper-machine web—specifically a nip press belt—isdescribed in which a carrier web with a polymer coating is provided,which comprises a thixotropic material (for example, aramid or silicaglass). The base material specified here, again, is polyurethane.

Finally, the patent DE 44 38 354 A1 discloses another press jacket madeof elastomeric material, in which is embedded substantially parallel tothe surface a woven layer of an extremely stable material, for examplearamid fibers.

The objective of the invention is to provide a nip press belt, theworking characteristics of which are improved in comparison to theseknown solutions, in particular with respect to optimized elasticityproperties and advantageous multidimensional bending behavior, so as toobtain quiet running of the machinery combined with low driving powerand a high total running time.

This objective is achieved by a nip press belt with the characteristicsgiven in claims 1 and 12. As a result of the invention an improved wetpress or an improved calender for the manufacture of paper, cardboardand tissue is also provided.

The invention includes the fundamental idea of constructing the nippress belt with an elastomer layer made of an extremely soft elastomerthat conforms readily to other surfaces (and in addition is sufficientlyeconomical), as a result of which the belt as a whole can be endowedwith an unusually low bending moment and a low overall modulus ofelasticity. Achievement of the above-mentioned working characteristics,improved in several respects, results from the combination of theseadvantageously adjusted parameters.

In one advantageous embodiment the soft-rubber elastomer layer of thebelt comprises a rubber compound that is highly abrasion-resistant,being made of several rubber or silicone-rubber composites. Inparticular, these composites are homogeneously mixed with one another inthe compound.

The choice of an appropriate combination or combinations of materialsallows the hardness of the soft-rubber layer to be adjusted according tothe requirements of the particular application, i.e. to suit thecustomer's desires. The hardness is preferably in the range between 20and 50 P+J, and is about 35 P+J in an embodiment currently regarded asthe standard variant.

In order to adjust the breaking strength of the belt to severe demands,a fiber reinforcement or an interlocking material is incorporated intothe elastomer layer. This textile reinforcing component, according tothe information currently available, preferably takes the form ofstrands oriented in the circumferential and/or longitudinal direction ofthe belt. It also seems reasonable to construct it as a nonwoven fabricof staple (short) fibers, either as an independent reinforcing layer orin combination with another type of reinforcement, such as theabove-mentioned strands. The employment of a woven fabric as reinforcinglayer is also possible.

As reinforcing material, in particular modified glass or carbon fiberscan be used, or especially the highly stable plastic fibers. In the lastcase polyester and polyamide copolymers or aramid are the primarycandidates. Depending on the customer's requirements, combinations ormixtures of these materials can usefully be employed.

The reinforcing layer is preferably incorporated near the back surfaceof the elastomer-textile composite, first in order not to impair theelastic properties of the front surface of the belt, and also to ensurea minimal bending moment at the curved part of the pressing element andat the ends of the belt, and thus to achieve the intended overalloptimization of the multidimensional curvature behavior.

Toward this end, the total thickness of the belt (chosen in accordancewith the elasticity and stability properties of the individualmaterials) will in particular be in the range between 5 and 10 mm, thestandard being ca. 7.5 mm.

The front surface of the belt can be made smooth, which will beparticularly appropriate for use in a calender. However, specificallyfor use in a wet press, it can also have a well-defined structure. Thestructure that seems preferable at present consists of recesses in theform of pocket holes. These can in particular be approximatelycylindrical in shape and isolated from one another. However, structuresin which recesses are connected to one another, grooved structures andthe like can also be useful.

Depending on the specific conditions of use, the open cross-sectionalarea of the structured front side (i.e., the total area occupied by therecesses) advantageously amounts to 10–50% of the total area of thefront surface of the belt. For conventional wet-press applications, itseems appropriate for the openings to have an area equal to about 20% ofthe total. In particular, the recesses have lateral dimensions in therange between 0.5 and 5 mm, in particular between 1 and 3 mm. Theirdepth is advantageously in the same range.

According to a further essential aspect of the invention, the nip pressbelt has a stiffness or—converted to take account of crosssection—bending stress distinctly below that of conventional belts. Thusthe force required to achieve a deflection of 15 mm in a three-pointbending test of a specimen 30 mm wide and 5 mm thick, set on supports100 mm apart, is preferably 17 N or less, in particular 13 N or less,and the bending stress is below 110 N/cm² and in particular below 90N/cm². In an embodiment of the belt in accordance with the inventionthat is preferred for practical purposes, a force of 11 N and a bendingstress of 70 N/cm² was measured.

With such elasticity it contributes substantially to a saving of drivingpower and to quiet running of the associated wet press or calender, andthis benefit is not offset by substantial restrictions with respect tothe service life of the belt. On the contrary, the reduction ofdeformation-dependent strain in the material actually has a positiveeffect on the working or service life of the belt.

Other advantages and useful aspects of the invention will be apparentfrom the subordinate claims, as well as from the following descriptionin outline of an exemplary embodiment with reference to the figures,wherein

FIG. 1 is a schematic drawing of the wet-press section of a papermachine in longitudinal section, and

FIG. 2 is a simplified cross-sectional drawing of an embodiment of thenip press belt of such a wet press, constructed in accordance with theinvention.

FIG. 1 shows part of the nip of a wet press 1 (nip press) of a papermachine with elongated nip. Opposite a pressing element 3 is disposed apress-roller 5, and in the gap between these two, namely the nip 7,water is removed from a paper web 13 enclosed between the surface of thepress-roller 5 and a nip press belt 9 that slides along the presssingelement 3, with a pulp felt 11.

The nip press belt 9 must on one hand have elasticity and flexibilitysuch that it conforms as well as possible to the curved surface of thepressing element 3 and exerts an elastic pressure uniformly upon thepaper web 13 (by way of the pulp felt 11). On the other hand, it must besufficiently stable to withstand the high, long-term stress (tension,pressure and vibration) in the nip 7 for a service time that iseconomically acceptable. The hardness of the soft rubber in the standarddesign is 35 P+J, but it can be varied within broad limits, as desiredby the customer, by appropriate variation and mixing of rubbercomposites. The soft rubber can be used in the long term at temperaturesabove 100° C., and can tolerate temperature peaks of up to ca. 130° C.

A nip press belt 9 suitable to meet these demands is shown(schematically) in cross section in FIG. 2. This nip press belt 9 has asits base a band of soft rubber 9 a, provided near its back surface witha textile reinforcement layer 9 b consisting of carbon fibers ormodified glass fibers or aramid fibers. The overall thickness of thebelt is 7.5 mm; in the region of the lateral lips 9 c it is ca. 5 mm. Inthe front surface of the belt are formed pocket holes 9 d with adiameter of 2.5 mm and a depth of 2 mm. The total area of the openingsamounts to 20% of the area of the front surface of the belt.

The embodiment of the invention is not restricted to this example, butis also possible in many modifications, with respect both to thematerials of which the individual components are made and to thegeometric dimensions.

LIST OF REFERENCE NUMERALS

-   1 Wet press-   3 Pressing element-   5 Press-roller-   7 Nip-   9 Nip press belt-   9 a Band of soft rubber-   9 b Textile reinforcing layer-   9 c Lip-   9 d Pocket hole-   11 Pulp felt-   13 Paper web

1. Nip press belt of a wet press or a calendar with elongated nip, inparticular for a paper, cardboard or tissue machine, with a flexibleelastomer layer that is impermeable to liquids and with a textilereinforcing layer, characterized in that the elastomer layer comprisessoft rubber with a hardness in the range between 5 and 100 P+J; andfurther characterized in that in a three-point bending test of aspecimen of the nip press belt 30 mm wide and 5 mm thick, set onto twosupports 100 mm apart, the force that must be exerted to produce adeflection of 15 mm is 17 N or less and the associated bending stress is110 N/cm² or less.
 2. Nip press belt according to claim 1, characterizedin that the elastomer layer comprises a highly abrasion-resistant rubbercompound of a plurality of rubber and/or silicone-rubber composites. 3.Nip press belt according to claim 1, characterized in that the hardnessof the elastomer layer is in the range between 20 and 50 P+J.
 4. Nippress belt according to claim 1, characterized in that the textilereinforcing layer is incorporated into the elastomer layer near thesurface of the latter that faces a pressing element during use, in sucha way that no substantial change is produced in the rubber-elasticproperties of the elastomer layer at its front surface.
 5. Nip pressbelt according to claim 1, characterized in that the textile reinforcinglayer comprises modified glass fibers, carbon fibers and/or highlystable plastic fibers.
 6. Nip press belt according to claim 1,characterized in that the textile reinforcing layer comprises threadsoriented in the circumferential direction of the nip press belt.
 7. Nippress belt according to claim 1, characterized in that the textilereinforcing layer comprises a nonwoven fabric of stable (short) fibers.8. Nip press belt according to claim 1, characterized in that thetextile reinforcing layer comprises a nonwoven fabric of staple (short)fibers.
 9. Nip press belt according to claim 1, characterized by astructured front surface with recesses.
 10. Nip press belt according toclaim 9, characterized in that the recesses have lateral dimensions anda depth in the range between 0.5 and 5 mm.
 11. Nip press belt accordingto claim 9, characterized in that the an open cross-sectional area ofthe structured front surface accounts for a proportion of between 10 and50% of the total area of the front surface.
 12. Nip press belt accordingto claim 1, characterized in that in the three-point test a force of 11N is to be exerted and the associated bending stress is 70 N/cm². 13.Wet press with elongated nip of a paper, cardboard or tissue machine,with a nip press belt according to claim
 1. 14. Calender with elongatednip of a paper, cardboard or tissue machine, with a nip press beltaccording to claim 1.